The present invention relates to a nuclear reactor and, more particularly, to a boiling water nuclear reactor having a storage basin with an improved gaging system for examination of nuclear fuel channel members.
In a boiling water reactor, nuclear fuel channel members are subjected to radiation exposure during the normal operation of the reactor. Radiation in the form of neutron flux, defined as the number of neutrons passing through a unit volume per unit of time, is not uniform over the length of the channel members. The lack of uniformity of the neutron flux is due to several factors including location of the channel member in the reactor core with respect to the core center and periphery, and the presence of adjacent control blades. Because the neutron flux is not uniform over the length of the channel members, anisotropic crystal growth results causing the channel members to grow longitudinally with the sides receiving the highest amount of radiation undergoing the greatest movement. The unequal growth causes bowing of the channel members along their longitudinal axes away from the high or maximum neutron flux regions. Additionally, deviations in cross section or swelling of the channel members appear to be caused by (1) action of fluid under pressure on the inside of the channel member, (2) temperature of the superheated water and (3) neutron flux, so that the channel members appear to swell or balloon outwardly.
In a boiling water nuclear reactor, the channel members direct the core coolant flow through and around each fuel bundle and also serve to guide the control rods. Excessive swelling or ballooning of the channel members near or adjacent to the lower end can result in improper flow of coolant through and around the fuel bundles causing overheating. The channel members also serve to guide the control rods or blades which must remain free to be moved in the close spaces provided for that purpose in the reactor core between adjacent channel members. Critical dimensions must therefore be maintained between adjacent channel members so as not to interfere with the insertion or withdrawal of the control rods in the reactor core. Such neutron flux induced dimensional changes in the channel members if left unchecked could result in a malfunction of the reactor. Also such radiation-induced changes occur at different rates depending upon the orientation and location of each channel member in the reactor core.
By determining the magnitude and direction of bowing, swelling and ballooning of each channel member, it has been found that an appropriate location within the reactor core could be selected so as to prolong the useful life of a given channel member. Extended life or use of channel members is achieved by moving them into an area of the reactor core where their longitudinal bow will not affect reactor performance.
There have hitherto been provided gages for the measurement of nuclear channel members. For example, one such device classified the channel members as acceptable or not acceptable with the result that all channel members found not acceptable were discarded. Other more complex and expensive measuring systems have been provided. For example, Weilbacher et al, U.S. Pat. No. 4,036,686 granted July 19, 1977 discloses a system for examination and measurement of nuclear fuel rods while submerged in the tank of a nuclear reactor which includes a movable carriage assembly with means for visual examination and for measurement of the fuel rods from a remote location, the carriage assembly traversing longitudinally from end to end along the fuel rods. Weilbacher, U.S. Pat. No. 4,048,009 granted Sept. 13, 1977 discloses a system for checking the straightness of the extensions of control rods of a nuclear reactor while submerged in a fuel storage pool by means of a vertically movable carriage with sensors which travel longitudinally from end to end along the control rod extensions including remote reading of the information outside the storage pool.